Start-up system for combined circulation steam generator



1968 F. J. HANZALEK ETAL 3,370,573

START-UP SYSTEM FOR COMBINED CIRCULATION STEAM GENERATOR Filed Dec. 12. 1966 FIG. 3

FURNACE WALL DEMINERALIZER HEATER 24 ER FEEDWATER ECONOMIZER 1*- I l 25 DEAERATOR FIG. I

\ I. P. FEEDWATER HEATER INVENTORS FREDERICK J. HANZALEK FIG. 2 ALWlN w. AMBROSE AGENT United States Patent C 3,370,573 START-UP SYSTEM FOR COMBINED CIRCULA- TRON STEAM GENERATOR Frederick J. Hanzalek, Suflield, and Alwin W. Ambrose,

Ha'zardvilie, Conn", assiguors to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Dec. 12, 1966, Ser. No. 600,905 9 Claims. (Cl. 122-406) ABSTRACT OF THE DISCLOSURE A system of starting a combined circulation steam generator under no-through-flow operating conditions by combining fluid recirculation around the furnace tubes circuit with overflow of the swell by way of the cold end of the boiler through the feed pump recirculating and pressure breakdown system, or through a separate swell relieving conduit leading to the low pressure region of the feedwater system.

Background of the invention The invention relates to a method and system of operating, and more specifically, of starting a high pressure forced through-flow steam generator of the type commonly referred to as a combined circulation steam generator. For a more detailed description of the characteristics of a combined circulation steam generator, reference may be had to US. Patent No. 3,194,217 issued to H. A. Grabowski on July 13, 1965, and entitled Boiler Cleanup Method for Combined Circulation Steam Generator.

While the working medium commonly employed in generators of the character indicated is water and steam derived therefrom, it is recognized that other specific fluid mediums may be employed in lieu thereof. Accordingly, the terms water and steam appearing in this description and in the appended claims are respectively understood to be the equivalent of other suitable liquids and vapors of such liquids.

When starting a forced through-flow high pressure steam generator, it is customary to separate the steam generating portion of the working fluid path from the steam heating or superheating portion by a so-called boiler throttle valve or BT valve although the present invention is also applicable to units which do not have such a valve. During one phase of the conventional cold start-up operation, the boiler is filled with the working fluid up to the BT valve which is closed and the pressure is raised to near normal operating pressure or other suitable pressure by the feed pump. As soon as heat is applied in the furnace to the furnace tubes, the volume of the fluid expands due to increase in temperature, which expansion causes an overflow of the fluid to the condenser or to other low pressure portion of the feed cycle through the so-called boiler extraction valve or BE valve provided in the fluid flow path downstream of the furnace wall tubes. In addition, in a conventional forced through flow boiler, a flow of the fluids through the furnace tubes and the BE valve of approximately 30% of the rated maximum flow must be maintained to protect these furnace tubes against excessive heat during the start-up operation.

In a combined circulation steam generator, however, such minimum through-flow of 30% is commonly reduced to approximately 10% by the provision of a fluid recirculating circuit around the furnace tubes. This maintains the required flow through the furnace tubes, but prevents the waste of heat which in a conventional forced through flow boiler occurs when most of the portion of the heat contained in the 30% of fluid is lost to the conice denser. Yet, in a combined circulation boiler the amount of fluid, including the overflow due to swell of the working fluid, customarily discharged through the BE valve, is still considerable. The breakdown of pressure, which for several hours must occur in the BE valve from a high operating pressure to a pressure approaching that prevailing in the low pressure portion of the feedwater system, causes inordinate wear of the BE valve and may require shutdown of the unit and costly repair of the valve after only a relatively short time of service. Such heavy wear subtracts from the usefulness of the BE valve in performing its main function, that is of maintaining a desired pressure in the steam evaporating furnace wall portion and/ or other portions of the boiler and of partic'ipating in the transfer of this function to the BT valve or turbine valve when the steam generator unit assumes normal operation.

Summary Costly maintenance due to breakdown of the boiler extraction valve during start-up operation of a combined circulation steam generator is avoided by establishing no-through-fiow conditions in the steam generator during an early start-up operation phase While recirculating the working fluid through the furnace tube circuits to protect these tubes against excessive heat, and by relief of the swell of the working fluid through the cold end of the boiler by way of the feed pump recirculating and pressure breakdown system, or by way of a separate swell relieving conduit leading to the low pressure region of the feedwater system.

Brief description of the drawing FIG. 1 depicts a diagrammatic representation of a forced through-flow steam generating plant of the combined circulation type employing the herein disclosed inventive swell relieving means at the cold end of the steam generator;

FIG. 2 is a diagrammatic representation of a portion of the steam generating plant of FIG. 1, however showing the swell relieving means being combined with the feedwater pump recirculating or bleed-off line; and

FIG. 3 is a typical pressure reduction device of the capillary tube type.

Description of the preferred embodiment The diagram of FIG. 1 shows a forced through-flow steam generator generally indicated by the numeral 10. The steam generator is adapted to be fired in any conventional manner by appropriate firing means (not shown), and to deliver high pressure high temperature steam to a steam utilization device such as steam turbine 12. Condensate pump 14 and feed pumps 16 and 18 force water or working fluid from a source of supply such as condenser 20, by way of demineralizer 22, low pressure feedwater heater 24, deaerator 26, intermediate pressure feedwater heater 27 into a first fluid flow section A which may comprise high pressure feedwater heater 28. Steam generator 10 includes a second fluid flow section B and a third fluid flow section C which are interconnected with fluid flow section A for series flow therethrough as shown. Flow section B preferably includes an economizer 30 and furnace wall tubes 32. Flow section C includes one or more superheater sections 34. Interpositioned between the flow sections B and C may be interposed a so-called boiler throttling valve BT which during normal operation of the steam generator remains I fully open. The feedwatcr, after having been preheated in the feedwater heaters 24, 27, and 28 and economizer 30, is converted into steam in furnace wall tubes 32 and after passing through valve ET the steam is superheated in flow section C from whence it is delivered to turbine 12 during normal operation of the steam generator.

When starting up the steam generator and turbine 12 valve ET is normally closed. During a later phase of the start-up operation, high pressure water at 3500 psi. pressure and 800 F. temperature, for example, is passed through conduit 35 and boiler extraction valve BE to a water and steam separator or flash tank 36 with the pressure thereby being reduced to the desired turbine start-up pressure such as 500 psi. Steam produced by flashing is separated from the water in separator 36 and flows through conduit 37 to superheater 34-tor further heating. The steam is then conducted by way of steam line 38 to turbine 12 for the heating, rolling, and starting of the turbine. The water separated from the steam in separator 36 is generally conducted to the condenser by way of pipe 39, or to any other feedwater storage vessel.

During an earlier phase of the conventional start-up procedure, when the pressure of the water is raised from the feedwater system pressure to rated operating pressure such as 3500 psi. and the temperature of the water to 550 F., for example, the throttling duty of the boiler extraction valve BB is very severe, causing extensive wear of the valve parts. This is due to the fact that, in addition to through flow which may be between 10 and of the maximum rated steam producing capacity of the unit, an overflow quantity must be passed through valve BE. As earlier stated, this overflow quantity results from the swelling of the working fluid when the volume thereof increases due to the heating of the fluid.

To eliminate the excessive wear of the valve BE during this early stage of the start-up procedure, the invention provides a method in which through flow is entirely eliminated be completely closing valve BE. To permit such no-through-flow operation, the quantity of the working fluid, which in a combined circulation steam generator is generally recirculated around the furnace tubes 32, or the furnace tubes 32 and economizer 30, by way of recirculating conduit 49 and recirculation pump 42, must be sufficient to protect the heating surface from overheating even without any through flow.

To make it possible to thus relieve the valve BE of the most severe throttling duty, the invention provides for the overflow caused by the swelling of the water during the pressure raising and heating phase, to be discharged by way of the relatively cold end of the steam generator such as through conduit 44 and one or several valves 46. This conduit 44 includes a pressure breakdown device which may consist of a series of orifices 48. The pressure loss through these orifices and through conduit 44 and valves 46 is limited to such a value that flashing to steam is confined to the receiving tank, such as deaerator 26 for example. It is important and very desirable to reduce the pressure in the swell relieving means 44, 46, and 48 at such a rate that no flashing occurs within these elements, in order to prevent backing up of the overflow, which would cause an upsetting of the temperature rate at which the fluid in the steam generator is being heated. The overflow may also be discharged into other portions of the feedwater system wherein a low pressure such as 5 p.s.i. prevails.

Other pressure breakdown devices, for example capillary tubing 50 as shown in FIG. 3, can be used in place of or in addition to orifices 48. This tubing may consist of one or several pipes having a small diameter bore 52, and being suitable length to produce the desired pressure reduction.

FIG. 2 shows an embodiment of the invention wherein the overflow or swell is discharged through the bypass customarily installed in the discharge line of the boiler feed pump. It is almost common knowledge that a pump operated atshut-otf would soon overheat and seize. Accordingly, some specific means must be applied to maintain a predetermined flow through the boiler feed pump regardless of through-flow. This is acornplished by recirculating a limited quantity of fluid through bypass line 54. This bypass is located on the pump side of the .dis-

charge gate and check valves 56 and leads to some region of low pressure in the boiler feed cycle. In the embodi ment of FIG. 2, line 54 leads to the deaerator 26. The capacity of the bypass is generally such that should the demand of the boiler fall to zero or should one of the valvesbecome closed while the pump is running at full speed, the flow through the pump will not fall below the predetermined permissible minimum.

In accordance with the invention the overflow due to swell is discharged into the pump bypass line 54 by way of conduit 58 and valve 69. The capacity of line 54 and the pressur'ebreakdown device 62 customarily used in bypass line 54, is in accordance with the invention increased to seri/e the two-fold purpose of (1) relieving the overflow due to swell of the fluid during the start-up operation, and (2) maintaining a predetermined minimum flow through thefeed pump 18 regardless of the delivery flow. As in the embodiment of FIG. 1, the pressure breakdown device can be of any suitable type such as a series of orifices 48 or capillary tubing 56.

While we have illustrated and described two preferred embodiments of our invention, it is to be understood that suchtare merely illustrative and not restrictive and that variations and modifications may be made'therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to availourselves of such changes as fall within the purview of our invention.

We claim:

1. In combination with a force through flow steam generating unit comprising pump means, a steam utilization device, fluid flow circuitry establishing communication between the pump means and the steam utilization device and including steam generating means having series connected first, second and third fluid flow sections, the pump means being connected to and adapted to transmit water at high pressure through the first and second flow sections wherein it is adapted to be converted into steam which is transmitted through the third flow section wherein it is adapted to be converted into superheated steam, recirculating circuit means for recirculating fluid from the outlet of the second fluid flow section of a first portion of the circuitry located downstream of the first section, stop valve means for arresting the flow of the fluid at a point downstream of the second fluid flow section during start-up operation of the steam generating unit, and a low pressure region in the boiler feed cycle receiving condensate from the steam utilizing device and communicating with the pump means, the improvement comprising a first conduit adapted for relieving fluid swell during a start-up operation by transmitting fluid from a second portion of the circuitry intermediate the outlet of the first fluid flow section and the inlet of the second fluid flow section to said low pressure portion of the boiler feed cycle.

2. Apparatus according to claim 1, wherein a pressure breakdown device is included in said first conduit, said breakdown device comprising a plurality of orifices adapted for series flow therethrough.

3. Apparatus according to claim 1, whereina pressure breakdown device is included in said first circuit, said breakdown device comprising capillary tubing.

4. Apparatus according to claim 1, wherein the first fluid flow section includes a high pressure feedwater heater and the second fluid flow'section an economizer, and wherein the swell relieving conduit means is adapted for receiving fluid from a point of departure located in a portion of the fluid flow circuitry downstream of the feedwater heater and upstream of the economizer.

5. Apparatus according to claim 4, wherein check valve means are provided in the fluid flow circuitry between said point of departure and the feedwater heater to prevent reverse fluid fl w back throu the feedwater heater.

6. Apparatus according to claim 1, wherein a stop and check valve is provided between said pump means and said first fluid flow section, and a bleed-0E conduit for bleeding ofl fluid from the discharge side of the pump means upstream of said stop and check valve to said low pressure region, and wherein said first conduit establishes communication between said second portion of the circuitry and said bleed-0S conduit.

7. Apparatus according to claim 6 wherein a pressure breakdown device is included in at least one of said first circuit and said bleed-off conduit.

8. The method of starting a forced through-flow steam generator having a working fluid flow path including a low pressure feedwater region, a feed pump, a feedwater heater, furnace wall heating surfaces, and a superheater, all adapted for series flow of the working fluid therethrough in the order named, the steps comprising:

(1) blocking the flow of the working fluid at a point between the furnace Wall heating surface and the superheater;

(2) increasing the pressure of the Working fluid to a predetermined operating pressure;

(3) recirculating a portion of the working fluid from the outlet of the furnace wall heating surface directly to the inlet thereof; and

(4) liberating heat in the furnace for heating the working fluid;

said method comprising the additional improvement step of:

(5) relieving the swell of the heated working fluid by flow of fluid from a point of said fluid flow path intermediate said feedwater heater and said furnace wall heating surfaces to said low pressure feedwater region, while reducing the pressure of the relieved fluid from the operating pressure to that of the low pressure feedwater region.

9. The method according to claim 8, including the ad ditional steps of:

(6) bleeding oflF a quantity of the working fluid from the discharge side of the feed pump to the low pressure feedwater region, while reducing the pressure of the bleed-off quantity from the pump discharge pressure to that of the low pressure feedwater region; and

(7) relieving the swell of the heated working fluid by adding to and mixing with the bleed-off quantity the overflow quantity resulting from the swell caused by the heating of the working fluid.

References Cited UNITED STATES PATENTS KENNETH W. SPRAGUE, Primary Examiner. 

